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Preparation and performance of spinel LiMn2O4 by a citrate route with combustion
Journal of Alloys and Compounds 352 (2003) 250–254
L
www.elsevier.com / locate / jallcom
Preparation and performance of spinel LiMn 2 O 4 by a citrate route with combustion K. Du*, H. Zhang Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China Received 19 February 2002; accepted 13 September 2002
Abstract Nano- and submicrometer LiMn 2 O 4 spinel particles were successfully synthesized by a modified citrate route with spontaneous combustion at 250 8C followed by a calcination process. Citric acid was used as the chelating and combustion-assistant agent. The synthesized LiMn 2 O 4 showed a high electrochemical performance compared with commercial LiMn 2 O 4 materials. The initial discharge specific capacity reached 130 mAh / g. The effects of the processing factors and element substitution on the microstructure and electrochemical performance of LiMn 2 O 4 were also examined. 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrode materials; Chemical synthesis; Electrochemical reactions
1. Introduction Spinel LiMn 2 O 4 is one of the most promising cathodic materials for Li-ion batteries owing to its lower cost, lighter toxicity and higher voltage compared with the layered LiCoO 2 -like oxides [1,2]. It can be synthesized by several methods. In a typical solid-state route [1,3], LiMn 2 O 4 is produced by solid-state reactions that involve the mechanical mixing of oxides and / or carbonates followed by extended grinding and calcining at high temperatures for a long time, since the formation of LiMn 2 O 4 with a homogeneous composition distribution requires longrange diffusion of the atoms in the reactants. Otherwise, abnormal grain growth and poor control of stoichiometry due to the nonhomogeneous composition distribution may lead to poor electrochemical performances. Solution technique is an alternative, in which all the components can be homogeneously distributed at the atomic–molecular level and the synthesis can be conducted at low temperatures for short periods. Recently a new route, called solution combustion, has been developed. Several kinds of combusting agents were used, such as poly(acrylic acid) [4], urea [5] and diformylhydrazine [6]. In this paper, it is *Corresponding author. Tel.: 186-625-11070-8936: fax: 186-322-00534. E-mail address: [email protected] (K. Du).
reported that LiMn 2 O 4 can be synthesized by a modified citrate route with spontaneous combustion, in which citric acid was used not only as a chelating agent like in some solution routes [7–9], but also as a combustion assistant agent. This technique is more time- and cost-saving than the sol–gel technique using citric acid. The effects of the processing factors on the microstructure and electrochemical performance were examined and preliminarily optimized. It was demonstrated that the electrochemical discharge capacity of LiMn 2 O 4 with nanoand submicrometer size by this combustion route reaches 130 mAh / g. Meanwhile, the influence of the element substitution of Me (Me5Co, Cr, Fe) for Mn on the electrochemical properties by this process was also determined. A potentiostatic current transient technique was used to investigate the transport characteristics of Li 1 in the LiMn 2 O 4 cathode, which is an important factor in determining the charge and discharge properties of the cathode, especially concerned with the rate behavior.
2. Experimental To prepare spinel LiMn 2 O 4 by the modified citrate route with spontaneous combustion, LiNO 3 and 50% aqueous solution of Mn(NO 3 ) 2 were homogeneously mixed in the appropriate Li:Mn atomic ratio and then were added to a
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-8388(02)01165-9
K. Du, H. Zhang / Journal of Alloys and Compounds 352 (2003) 250–254
mixed solution of citric acid and ethylene glycol. The resulting solution was heated to about 90 8C, when a reaction occurred violently, and the pH value of the solution was then adjusted to about 7 by ammonia solution. The water and organic compound were then removed by heating the solution in an electric cooker, at about 250 8C—a spontaneous combustion without flame happened and a dark-grey plumy precursor was quickly formed. Calcining the precursor in air for an appropriate temperature and time, the LiMn 2 O 4 with structure spinel was synthesized. By mixing the proper quantity of Me(NO 3 ) n (Me5Co, Cr, Fe) with LiNO 3 and Mn(NO 3 ) 2 , the substituted compounds of LiMe x Mn 22x O 4 (x50.1 and 0.2) were prepared. The structure of LiMn 2 O 4 obtained was examined by X-ray diffraction (XRD) with CuKa radiation. The composition was analyzed with inductively-coupled plasma (ICP)-AES. The primary particle size and morphological characteristics were observed by transmission electron microscopy (TEM). The phase transition near ambient temperature was examined by DC resistance measurement. Electrochemical measurements were carried out by using coin-type cells (2025) with LiMn 2 O 4 or LiMe x Mn 22x O 4 –acetylene black–PTFE (85:10:5, w / w) as cathode, Li foil as the anode, 1 M LiClO 4 –EC1DME (50:50, v / v) as electrolyte, and a poly(ethylene–co-propylene) membrane as the separator. The cell was assembled in a glove box filled with pure Ar and charge– discharged on an Arbin instrument with current density of 0.26 mA / cm 2 in the voltage range between 3.0 and 4.2 V at room temperature. Two-electrode electrochemical cells were employed for cyclic voltammetry from 3.0 to 4.5 V at a scanning rate of 0.2 mV/ s and for examining the transport of Li 1 by potentiostatic current transient technique on a Solartron SI 1287 electrochemical interface.
251
Fig. 1. XRD patterns of the precursor and products synthesized at different calcination temperature.
peaks of LiMn 2 O 4 become sharper with increasing calcination temperature and this reveals that with the temperature rising, the crystal of LiMn 2 O 4 becomes more perfect and the grain size becomes larger. The pure phase of spinel LiMn 2 O 4 was obtained by calcining at temperatures above 800 8C. At temperatures beyond 850 8C, LiMn 2 O 4 would decompose. It is commonly accepted that the LiMn 2 O 4 with a pure spinel phase has excellent electrochemical performance, especially discharge capacity, for 4 V Li-ion battery cathode. The pH value of the solution affects the distribution of Mn and Li atoms and thus the structure of the final product. The XRD plots of the products obtained for pH 2, 7 and 9 are shown in Fig. 2. The XRD pattern at pH 9 is more complicated, which indicates that there is a phase other than spinel LiMn 2 O 4 . The composition of the product prepared at pH 7 and calcined at 800 8C is Li:Mn50.956:2 by ICP analysis. The ˚ lattice parameter is a58.2506 A. The TEM picture in Fig. 3 shows that the primary particle of LiMn 2 O 4 prepared by this process is fine, 10–100 nm, i.e. nanometer–micrometer size. However the
3. Results and discussion
3.1. Structure characteristics Fig. 1a shows XRD patterns of the precursor, which was obtained from the Li–Mn-citrate solution with pH 7 by combusting spontaneously at about 250 8C. Fig. 1b–g give the XRD pattern of the products, which were obtained by calcining the precursor at 400, 500, 600, 700, 800 and 850 8C for 22 h in air, respectively. Obviously, the precursor already has the spinel structure (Fig. 1a), because it was obtained under about 250 8C where a deformed spinel structure was formed by the short-range diffusion and rearrangement of the component atoms. For the product which was calcined at temperatures below 800 8C, a small peak clearly appears at 2u 5338, which is a characteristic peak of Mn 2 O 3 . Fig. 1 also indicates that the
Fig. 2. XRD patterns of products synthesized from the solution with different pH values.
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3.2. Electrochemical performance
Fig. 3. TEM picture of the particle.
agglomerate size measured by a granulometer is 1–100 mm for the most, which is almost 10 2 –10 3 larger than the size of the primary particle. It shows that there is a certain agglomeration in the LiMn 2 O 4 particles. Fig. 4 indicates the variation of DC resistance with temperature at 200–300 K. Below 270 K the R–T curve with semiconducting behavior can be well described by the small polaron hopping mechanism R ~ 1 /T exp (2E /kT ), where E is activation energy, T is temperature, k is Boltzmann constant. The obtained E50.4 eV value for the carrier transfer is comparable with other results [10]. Interestingly, there is an obvious inflexion around 270 K on the R–T curve, which may indicate that a phase transition occurs here. Such a structural phase transition near the ambient temperature for LiMn 2 O 4 was reported by others [11].
Fig. 4. DC resistance versus temperature curve.
Since the capacity of the Li foil anode is designed to be over, the variation of the electrochemical properties is attributed to the cathode of LiMn 2 O 4 . Fig. 5 shows the cyclic voltammograms of the 1st, 20th and 40th for the synthesized LiMn 2 O 4 with the sweeping rate 0.2 mV/ s. This figure shows two typical oxidation and two typical reduction peaks for LiMn 2 O 4 . This implies that Li 1 intercalation occurs in two processes. The two oxidation peaks are at 4.15 and 4.30 V, and the two reduction peaks are at 3.85 and 4.00 V. The ratio of the area enclosed by these three curves is 100:78:76 corresponding to 1st, 20th and 40th cycle under used applied sweeping rate. It means that the capacity loss occurs mostly in the first several cycles. The cycle performance of the galvanostatic discharge capacity in the first 20 cycles for the assembled coin-type cell is shown in Fig. 6. The discharge specific capacity of the product is 130, 106 and 98 mAh / g for the 1st, 20th and 50th cycles, respectively. For comparison, a coin-type cell with a commercial LiMn 2 O 4 was assembled using the same battery technique and processing, of which the cyclability performance is also shown in Fig. 6. The initial specific capacity of LiMn 2 O 4 prepared by this route is much higher than that of the commercial one and is comparable to those produced by normal citric processes [7–9].
3.3. Electrochemical performance of substituted LiMe x Mn22 x O4 As shown in Fig. 6, although the obtained LiMn 2 O 4 has a high initial specific capacity, the fading rate in the cycle curve could be further improved. For this, element doping and substituting are commonly considered to be an effective method. One of the advantages of the technique here, as all solution techniques, is that they can achieve a
Fig. 5. Cyclic voltammetry curves of the 1st, 20th and 40th cycles (scanning rate: 0.2 mV/ s).
K. Du, H. Zhang / Journal of Alloys and Compounds 352 (2003) 250–254
253
Fig. 6. Discharge cyclability curves (a) LiMn 2 O 4 prepared by this route with spontaneous combustion; (b) a commercial LiMn 2 O 4 by solid state route.
homogeneous distribution of components at the atomic– molecular level which is very beneficial to the homogeneous distribution of doped and substituted elements. The powders of LiMe x Mn 22x O 4 (Me: Co, Cr, Fe, x50.1, 0.2) and the 2025 coin-type cells with LiMe x Mn 22x O 4 cathode were prepared by the same technique as above. Fig. 7 illustrates the discharge specific capacity of the first 20 cycles. All the cyclic performances for three substituted LiMe x Mn 22x O 4 are improved at a certain expense of an initial discharge specific capacity. Among them, Cr is the most effective substituting element. The initial discharge specific capacity of LiCr 0.1 Mn 1.9 O 4 is about 124 mAh / g, only 4.6% loss compared with unsubstituted LiMn 2 O 4 , but the cyclic performance is much improved, for example, it keeps 116 mAh / g after 20 cycles, which is higher than 107 mAh / g for unsubstituted one. The efficiency of Cr doping was also reported with other techniques [12,13]. Usually it is accepted that Cr substituting for Mn can stabilize the spinel structure and thus improve the cycle performance.
Fig. 7. Discharge cyclability curves for LiMe x Mn 22x O 4 (a) x50.2; (b) x50.1.
transient curves for different applied potentials were obtained. Obviously, as shown in Fig. 9, the potentiostatic current transient cannot be well understood in the frame of
3.4. Transport of Li 1 in LiMn2 O4 spinel The discharge curves of the two-electrode electrochemical cell with LiMn 2 O 4 as cathode are shown in Fig. 8; the two discharge potential plateaus lie at 4.08 and 3.95 V. This implies that there are two intercalating steps for lithium intercalating, which are commonly accepted to correspond to two types of 8a position. Fig. 8 shows the potentiostatic current transient curves of the composite LiMn 2 O 4 cathode. The curve was obtained as following: the cell was previously charge– discharged for three cycles between 3.30 and 4.30 V to eliminate the effect of the initial irreversible capacity, then maintained in open circuit state at 4.30 V for 2 h, and the potentiostatic current transient was measured at a dropped potential. Repeating the above procedure, the current
Fig. 8. Discharge curves of the two-electrode electrochemical cell.
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K. Du, H. Zhang / Journal of Alloys and Compounds 352 (2003) 250–254
Fig. 9. Cathodic current transients at the various potential drops from 4.3 V.
Cottrell equation and there are three types of curves depending on the applied potential. For the first type, when the applied potential is higher than the first discharge plateau potential of 4.08 V, the absolute value of the slope of the logarithmic current transients increases monotonically with time during the whole intercalation. For the second one, when the applied potential locates between the first and second discharge plateau, i.e. between 4.08 and 3.95 V, a ‘current plateau’ to 2000 s followed by a sudden decrease of logarithmic current transient was observed. For the third one, the applied potential is dropped down to lower than the second discharge plateau of 3.95 V, the current decays with two ‘quasi-current plateau’ followed by rapidly decreasing and, interestingly, the first ‘quasicurrent-plateau’ becomes short with the increase of the dropped potential step. Those complicated behaviors of the potentiostatic current transient were also reported in some other oxide electrodes, e.g. LiCoO 2 [14–16], LiNiO 2 [17] and were well understood by the pure diffusion controlled model and cell-impedance-controlled model in single and / or double phase depending on the applied potential [15]. Certainly the observed potentiostatic current transient behavior with the applied potential may be roughly understood in the frame of the above models because the phase system corresponding to the applied potential is different and thus the cathode is different, in which Li 1 transports. For example, for the case of the applied potential higher than 3.95 V, the current decay occurs in the single phase of l-Mn 2 O 4 type under the ‘cell-impedance-controlled’ constraint. However, for complete understanding of the cause of the reported behavior, further detailed investigation is needed.
4. Conclusions A modified citrate route with spontaneous combustion at 250 8C followed by calcination at high temperature has
been successfully used to synthesize spinel LiMn 2 O 4 powder. The effects of processing factors on the microstructure and electrochemical performances were examined. The spinel LiMn 2 O 4 with single phase was obtained for solution with pH#7 by calcining around 800 8C. The powder is fine, the primary particle size is about 10–100 nm and agglomerate size is around 1–100 mm. It has an excellent initial specific capacity of 130 mAh / g but specific capacity drops to 107 mAh / g after 20 cycles. The substitution of Cr, Co, Fe for Mn was also conducted by this route. It was found that the cyclability is improved by substitution at a certain expense of the initial discharge specific capacity, for example, LiCr 0.1 Mn 1.9 O 4 has an initial discharge specific capacity of 124 mAh / g and keeps 116 mAh / g after 20 cycles. Three types of current transient-time behaviors depending on different applied potential were observed, which could not be understood by the Cottrell equation.
References [1] M.M. Thackeray, P.J. Johnson, L.A. Picciotto, Mat. Res. Bull. 19 (1984) 179–187. [2] J.M. Tarascon, E. Wang, F. Shokoohi, W.R. Makinnon, S. Colson, J. Electrochem. Soc. 138 (1991) 2859. [3] M.V. Banov, B. Amomchilov et al., J. Power Sources 57 (1995) 9. [4] H. Park, J. Kim, C. Lee, J. Power Sources 92 (2001) 124. [5] E.I. Santiago, S.T. Amancio-Filho, P.R. Bueno, L.O.S. Bulhoes, J. Power Sources 97–98 (2001) 447–449. [6] S. Rodrigues, N. Munichandraiah, A.K. Shukla, J. Power Sources 102 (2001) 322–325. [7] J.H. Choy, D.H. Kim, C.W. Kwon, S.J. Hwang, Y.I. Kim, J. Power Sources 77 (1999) 1. [8] W. Yang, Q. Liu, W. Qiu, S. Lu, L. Yang, Solid State Ionics 121 (1999) 79–84. [9] M. Yoshio, H. Noguchi, T. Miyashita et al., J. Power Sources 54 (1995) 483. [10] J. Sugiyama, T. Tamura, H. Yamauchi, J. Phys. Condens. Matter. 7 (1995) 9755. [11] J. Sugiyama, T. Atsumi, T. Hioki, S. Noda, N. Kamegashira, J. Alloy. Comp. 235 (1996) 163. [12] L. Hernan, J. Morales, L. Sanchez, J. Santos, Solid State Ionics 118 (1999) 179. [13] A.D. Robertson, S.H. Lu, W.F. Averill, W.F. Howard Jr., J. Electrochem Soc. 144 (1997) 3500. [14] Su-Il Pyun, Y.-M. Choi, J. Power Sources 68 (1997) 524–529. [15] H.-C. Shin, Su-Il Pyun, Electrochim. Acta 45 (1999) 489–501. [16] H.-C. Shin, Su-Il Pyun, S.-W. Kim, M.-H. Lee, Electrochim. Acta 46 (2001) 897–906. [17] M.-H. Lee, Su-Il Yun, H.-C. Shin, Solid State Ionics 140 (2001) 35–43.
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Preparation and performance of spinel LiMn 2 O 4 by a citrate route with combustion K. Du*, H. Zhang Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China Received 19 February 2002; accepted 13 September 2002
Abstract Nano- and submicrometer LiMn 2 O 4 spinel particles were successfully synthesized by a modified citrate route with spontaneous combustion at 250 8C followed by a calcination process. Citric acid was used as the chelating and combustion-assistant agent. The synthesized LiMn 2 O 4 showed a high electrochemical performance compared with commercial LiMn 2 O 4 materials. The initial discharge specific capacity reached 130 mAh / g. The effects of the processing factors and element substitution on the microstructure and electrochemical performance of LiMn 2 O 4 were also examined. 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrode materials; Chemical synthesis; Electrochemical reactions
1. Introduction Spinel LiMn 2 O 4 is one of the most promising cathodic materials for Li-ion batteries owing to its lower cost, lighter toxicity and higher voltage compared with the layered LiCoO 2 -like oxides [1,2]. It can be synthesized by several methods. In a typical solid-state route [1,3], LiMn 2 O 4 is produced by solid-state reactions that involve the mechanical mixing of oxides and / or carbonates followed by extended grinding and calcining at high temperatures for a long time, since the formation of LiMn 2 O 4 with a homogeneous composition distribution requires longrange diffusion of the atoms in the reactants. Otherwise, abnormal grain growth and poor control of stoichiometry due to the nonhomogeneous composition distribution may lead to poor electrochemical performances. Solution technique is an alternative, in which all the components can be homogeneously distributed at the atomic–molecular level and the synthesis can be conducted at low temperatures for short periods. Recently a new route, called solution combustion, has been developed. Several kinds of combusting agents were used, such as poly(acrylic acid) [4], urea [5] and diformylhydrazine [6]. In this paper, it is *Corresponding author. Tel.: 186-625-11070-8936: fax: 186-322-00534. E-mail address: [email protected] (K. Du).
reported that LiMn 2 O 4 can be synthesized by a modified citrate route with spontaneous combustion, in which citric acid was used not only as a chelating agent like in some solution routes [7–9], but also as a combustion assistant agent. This technique is more time- and cost-saving than the sol–gel technique using citric acid. The effects of the processing factors on the microstructure and electrochemical performance were examined and preliminarily optimized. It was demonstrated that the electrochemical discharge capacity of LiMn 2 O 4 with nanoand submicrometer size by this combustion route reaches 130 mAh / g. Meanwhile, the influence of the element substitution of Me (Me5Co, Cr, Fe) for Mn on the electrochemical properties by this process was also determined. A potentiostatic current transient technique was used to investigate the transport characteristics of Li 1 in the LiMn 2 O 4 cathode, which is an important factor in determining the charge and discharge properties of the cathode, especially concerned with the rate behavior.
2. Experimental To prepare spinel LiMn 2 O 4 by the modified citrate route with spontaneous combustion, LiNO 3 and 50% aqueous solution of Mn(NO 3 ) 2 were homogeneously mixed in the appropriate Li:Mn atomic ratio and then were added to a
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-8388(02)01165-9
K. Du, H. Zhang / Journal of Alloys and Compounds 352 (2003) 250–254
mixed solution of citric acid and ethylene glycol. The resulting solution was heated to about 90 8C, when a reaction occurred violently, and the pH value of the solution was then adjusted to about 7 by ammonia solution. The water and organic compound were then removed by heating the solution in an electric cooker, at about 250 8C—a spontaneous combustion without flame happened and a dark-grey plumy precursor was quickly formed. Calcining the precursor in air for an appropriate temperature and time, the LiMn 2 O 4 with structure spinel was synthesized. By mixing the proper quantity of Me(NO 3 ) n (Me5Co, Cr, Fe) with LiNO 3 and Mn(NO 3 ) 2 , the substituted compounds of LiMe x Mn 22x O 4 (x50.1 and 0.2) were prepared. The structure of LiMn 2 O 4 obtained was examined by X-ray diffraction (XRD) with CuKa radiation. The composition was analyzed with inductively-coupled plasma (ICP)-AES. The primary particle size and morphological characteristics were observed by transmission electron microscopy (TEM). The phase transition near ambient temperature was examined by DC resistance measurement. Electrochemical measurements were carried out by using coin-type cells (2025) with LiMn 2 O 4 or LiMe x Mn 22x O 4 –acetylene black–PTFE (85:10:5, w / w) as cathode, Li foil as the anode, 1 M LiClO 4 –EC1DME (50:50, v / v) as electrolyte, and a poly(ethylene–co-propylene) membrane as the separator. The cell was assembled in a glove box filled with pure Ar and charge– discharged on an Arbin instrument with current density of 0.26 mA / cm 2 in the voltage range between 3.0 and 4.2 V at room temperature. Two-electrode electrochemical cells were employed for cyclic voltammetry from 3.0 to 4.5 V at a scanning rate of 0.2 mV/ s and for examining the transport of Li 1 by potentiostatic current transient technique on a Solartron SI 1287 electrochemical interface.
251
Fig. 1. XRD patterns of the precursor and products synthesized at different calcination temperature.
peaks of LiMn 2 O 4 become sharper with increasing calcination temperature and this reveals that with the temperature rising, the crystal of LiMn 2 O 4 becomes more perfect and the grain size becomes larger. The pure phase of spinel LiMn 2 O 4 was obtained by calcining at temperatures above 800 8C. At temperatures beyond 850 8C, LiMn 2 O 4 would decompose. It is commonly accepted that the LiMn 2 O 4 with a pure spinel phase has excellent electrochemical performance, especially discharge capacity, for 4 V Li-ion battery cathode. The pH value of the solution affects the distribution of Mn and Li atoms and thus the structure of the final product. The XRD plots of the products obtained for pH 2, 7 and 9 are shown in Fig. 2. The XRD pattern at pH 9 is more complicated, which indicates that there is a phase other than spinel LiMn 2 O 4 . The composition of the product prepared at pH 7 and calcined at 800 8C is Li:Mn50.956:2 by ICP analysis. The ˚ lattice parameter is a58.2506 A. The TEM picture in Fig. 3 shows that the primary particle of LiMn 2 O 4 prepared by this process is fine, 10–100 nm, i.e. nanometer–micrometer size. However the
3. Results and discussion
3.1. Structure characteristics Fig. 1a shows XRD patterns of the precursor, which was obtained from the Li–Mn-citrate solution with pH 7 by combusting spontaneously at about 250 8C. Fig. 1b–g give the XRD pattern of the products, which were obtained by calcining the precursor at 400, 500, 600, 700, 800 and 850 8C for 22 h in air, respectively. Obviously, the precursor already has the spinel structure (Fig. 1a), because it was obtained under about 250 8C where a deformed spinel structure was formed by the short-range diffusion and rearrangement of the component atoms. For the product which was calcined at temperatures below 800 8C, a small peak clearly appears at 2u 5338, which is a characteristic peak of Mn 2 O 3 . Fig. 1 also indicates that the
Fig. 2. XRD patterns of products synthesized from the solution with different pH values.
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3.2. Electrochemical performance
Fig. 3. TEM picture of the particle.
agglomerate size measured by a granulometer is 1–100 mm for the most, which is almost 10 2 –10 3 larger than the size of the primary particle. It shows that there is a certain agglomeration in the LiMn 2 O 4 particles. Fig. 4 indicates the variation of DC resistance with temperature at 200–300 K. Below 270 K the R–T curve with semiconducting behavior can be well described by the small polaron hopping mechanism R ~ 1 /T exp (2E /kT ), where E is activation energy, T is temperature, k is Boltzmann constant. The obtained E50.4 eV value for the carrier transfer is comparable with other results [10]. Interestingly, there is an obvious inflexion around 270 K on the R–T curve, which may indicate that a phase transition occurs here. Such a structural phase transition near the ambient temperature for LiMn 2 O 4 was reported by others [11].
Fig. 4. DC resistance versus temperature curve.
Since the capacity of the Li foil anode is designed to be over, the variation of the electrochemical properties is attributed to the cathode of LiMn 2 O 4 . Fig. 5 shows the cyclic voltammograms of the 1st, 20th and 40th for the synthesized LiMn 2 O 4 with the sweeping rate 0.2 mV/ s. This figure shows two typical oxidation and two typical reduction peaks for LiMn 2 O 4 . This implies that Li 1 intercalation occurs in two processes. The two oxidation peaks are at 4.15 and 4.30 V, and the two reduction peaks are at 3.85 and 4.00 V. The ratio of the area enclosed by these three curves is 100:78:76 corresponding to 1st, 20th and 40th cycle under used applied sweeping rate. It means that the capacity loss occurs mostly in the first several cycles. The cycle performance of the galvanostatic discharge capacity in the first 20 cycles for the assembled coin-type cell is shown in Fig. 6. The discharge specific capacity of the product is 130, 106 and 98 mAh / g for the 1st, 20th and 50th cycles, respectively. For comparison, a coin-type cell with a commercial LiMn 2 O 4 was assembled using the same battery technique and processing, of which the cyclability performance is also shown in Fig. 6. The initial specific capacity of LiMn 2 O 4 prepared by this route is much higher than that of the commercial one and is comparable to those produced by normal citric processes [7–9].
3.3. Electrochemical performance of substituted LiMe x Mn22 x O4 As shown in Fig. 6, although the obtained LiMn 2 O 4 has a high initial specific capacity, the fading rate in the cycle curve could be further improved. For this, element doping and substituting are commonly considered to be an effective method. One of the advantages of the technique here, as all solution techniques, is that they can achieve a
Fig. 5. Cyclic voltammetry curves of the 1st, 20th and 40th cycles (scanning rate: 0.2 mV/ s).
K. Du, H. Zhang / Journal of Alloys and Compounds 352 (2003) 250–254
253
Fig. 6. Discharge cyclability curves (a) LiMn 2 O 4 prepared by this route with spontaneous combustion; (b) a commercial LiMn 2 O 4 by solid state route.
homogeneous distribution of components at the atomic– molecular level which is very beneficial to the homogeneous distribution of doped and substituted elements. The powders of LiMe x Mn 22x O 4 (Me: Co, Cr, Fe, x50.1, 0.2) and the 2025 coin-type cells with LiMe x Mn 22x O 4 cathode were prepared by the same technique as above. Fig. 7 illustrates the discharge specific capacity of the first 20 cycles. All the cyclic performances for three substituted LiMe x Mn 22x O 4 are improved at a certain expense of an initial discharge specific capacity. Among them, Cr is the most effective substituting element. The initial discharge specific capacity of LiCr 0.1 Mn 1.9 O 4 is about 124 mAh / g, only 4.6% loss compared with unsubstituted LiMn 2 O 4 , but the cyclic performance is much improved, for example, it keeps 116 mAh / g after 20 cycles, which is higher than 107 mAh / g for unsubstituted one. The efficiency of Cr doping was also reported with other techniques [12,13]. Usually it is accepted that Cr substituting for Mn can stabilize the spinel structure and thus improve the cycle performance.
Fig. 7. Discharge cyclability curves for LiMe x Mn 22x O 4 (a) x50.2; (b) x50.1.
transient curves for different applied potentials were obtained. Obviously, as shown in Fig. 9, the potentiostatic current transient cannot be well understood in the frame of
3.4. Transport of Li 1 in LiMn2 O4 spinel The discharge curves of the two-electrode electrochemical cell with LiMn 2 O 4 as cathode are shown in Fig. 8; the two discharge potential plateaus lie at 4.08 and 3.95 V. This implies that there are two intercalating steps for lithium intercalating, which are commonly accepted to correspond to two types of 8a position. Fig. 8 shows the potentiostatic current transient curves of the composite LiMn 2 O 4 cathode. The curve was obtained as following: the cell was previously charge– discharged for three cycles between 3.30 and 4.30 V to eliminate the effect of the initial irreversible capacity, then maintained in open circuit state at 4.30 V for 2 h, and the potentiostatic current transient was measured at a dropped potential. Repeating the above procedure, the current
Fig. 8. Discharge curves of the two-electrode electrochemical cell.
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Fig. 9. Cathodic current transients at the various potential drops from 4.3 V.
Cottrell equation and there are three types of curves depending on the applied potential. For the first type, when the applied potential is higher than the first discharge plateau potential of 4.08 V, the absolute value of the slope of the logarithmic current transients increases monotonically with time during the whole intercalation. For the second one, when the applied potential locates between the first and second discharge plateau, i.e. between 4.08 and 3.95 V, a ‘current plateau’ to 2000 s followed by a sudden decrease of logarithmic current transient was observed. For the third one, the applied potential is dropped down to lower than the second discharge plateau of 3.95 V, the current decays with two ‘quasi-current plateau’ followed by rapidly decreasing and, interestingly, the first ‘quasicurrent-plateau’ becomes short with the increase of the dropped potential step. Those complicated behaviors of the potentiostatic current transient were also reported in some other oxide electrodes, e.g. LiCoO 2 [14–16], LiNiO 2 [17] and were well understood by the pure diffusion controlled model and cell-impedance-controlled model in single and / or double phase depending on the applied potential [15]. Certainly the observed potentiostatic current transient behavior with the applied potential may be roughly understood in the frame of the above models because the phase system corresponding to the applied potential is different and thus the cathode is different, in which Li 1 transports. For example, for the case of the applied potential higher than 3.95 V, the current decay occurs in the single phase of l-Mn 2 O 4 type under the ‘cell-impedance-controlled’ constraint. However, for complete understanding of the cause of the reported behavior, further detailed investigation is needed.
4. Conclusions A modified citrate route with spontaneous combustion at 250 8C followed by calcination at high temperature has
been successfully used to synthesize spinel LiMn 2 O 4 powder. The effects of processing factors on the microstructure and electrochemical performances were examined. The spinel LiMn 2 O 4 with single phase was obtained for solution with pH#7 by calcining around 800 8C. The powder is fine, the primary particle size is about 10–100 nm and agglomerate size is around 1–100 mm. It has an excellent initial specific capacity of 130 mAh / g but specific capacity drops to 107 mAh / g after 20 cycles. The substitution of Cr, Co, Fe for Mn was also conducted by this route. It was found that the cyclability is improved by substitution at a certain expense of the initial discharge specific capacity, for example, LiCr 0.1 Mn 1.9 O 4 has an initial discharge specific capacity of 124 mAh / g and keeps 116 mAh / g after 20 cycles. Three types of current transient-time behaviors depending on different applied potential were observed, which could not be understood by the Cottrell equation.
References [1] M.M. Thackeray, P.J. Johnson, L.A. Picciotto, Mat. Res. Bull. 19 (1984) 179–187. [2] J.M. Tarascon, E. Wang, F. Shokoohi, W.R. Makinnon, S. Colson, J. Electrochem. Soc. 138 (1991) 2859. [3] M.V. Banov, B. Amomchilov et al., J. Power Sources 57 (1995) 9. [4] H. Park, J. Kim, C. Lee, J. Power Sources 92 (2001) 124. [5] E.I. Santiago, S.T. Amancio-Filho, P.R. Bueno, L.O.S. Bulhoes, J. Power Sources 97–98 (2001) 447–449. [6] S. Rodrigues, N. Munichandraiah, A.K. Shukla, J. Power Sources 102 (2001) 322–325. [7] J.H. Choy, D.H. Kim, C.W. Kwon, S.J. Hwang, Y.I. Kim, J. Power Sources 77 (1999) 1. [8] W. Yang, Q. Liu, W. Qiu, S. Lu, L. Yang, Solid State Ionics 121 (1999) 79–84. [9] M. Yoshio, H. Noguchi, T. Miyashita et al., J. Power Sources 54 (1995) 483. [10] J. Sugiyama, T. Tamura, H. Yamauchi, J. Phys. Condens. Matter. 7 (1995) 9755. [11] J. Sugiyama, T. Atsumi, T. Hioki, S. Noda, N. Kamegashira, J. Alloy. Comp. 235 (1996) 163. [12] L. Hernan, J. Morales, L. Sanchez, J. Santos, Solid State Ionics 118 (1999) 179. [13] A.D. Robertson, S.H. Lu, W.F. Averill, W.F. Howard Jr., J. Electrochem Soc. 144 (1997) 3500. [14] Su-Il Pyun, Y.-M. Choi, J. Power Sources 68 (1997) 524–529. [15] H.-C. Shin, Su-Il Pyun, Electrochim. Acta 45 (1999) 489–501. [16] H.-C. Shin, Su-Il Pyun, S.-W. Kim, M.-H. Lee, Electrochim. Acta 46 (2001) 897–906. [17] M.-H. Lee, Su-Il Yun, H.-C. Shin, Solid State Ionics 140 (2001) 35–43.